scholarly journals Low temperature gas phase reaction rate coefficient measurements: Toward modeling of stellar winds and the interstellar medium

2019 ◽  
Vol 15 (S350) ◽  
pp. 382-383
Author(s):  
Niclas A. West ◽  
Edward Rutter ◽  
Mark A. Blitz ◽  
Leen Decin ◽  
Dwayne E. Heard
Keyword(s):  
Low Temperature ◽  
Interstellar Medium ◽  
Gas Phase ◽  
Low Temperatures ◽  
Reaction Rate ◽  
Rate Coefficient ◽  
Building Blocks ◽  
Rate Coefficients ◽  
Stellar Winds ◽  
Phase Reaction

AbstractStellar winds of Asymptotic Giant Branch (AGB) stars are responsible for the production of ∼85% of the gas molecules in the interstellar medium (ISM), and yet very few of the gas phase rate coefficients under the relevant conditions (10 – 3000 K) needed to model the rate of production and loss of these molecules in stellar winds have been experimentally measured. If measured at all, the value of the rate coefficient has often only been obtained at room temperature, with extrapolation to lower and higher temperatures using the Arrhenius equation. However, non-Arrhenius behavior has been observed often in the few measured rate coefficients at low temperatures. In previous reactions studied, theoretical simulations of the formation of long-lived pre-reaction complexes and quantum mechanical tunneling through the barrier to reaction have been utilized to fit these non-Arrhenius behaviours of rate coefficients.Reaction rate coefficients that were predicted to produce the largest change in the production/loss of Complex Organic Molecules (COMs) in stellar winds at low temperatures were selected from a sensitivity analysis. Here we present measurements of rate coefficients using a pulsed Laval nozzle apparatus with the Pump Laser Photolysis - Laser Induced Fluorescence (PLP-LIF) technique. Gas flow temperatures between 30 – 134 K have been produced by the University of Leeds apparatus through the controlled expansion of N2 or Ar gas through Laval nozzles of a range of Mach numbers between 2.49 and 4.25.Reactions of interest include those of OH, CN, and CH with volatile organic species, in particular formaldehyde, a molecule which has been detected in the ISM. Kinetics measurements of these reactions at low temperatures will be presented using the decay of the radical reagent. Since formaldehyde and the formal radical (HCO) are potential building blocks of COMs in the interstellar medium, low temperature reaction rate coefficients for their production and loss can help to predict the formation pathways of COMs observed in the interstellar medium.

scholarly journals Pressure dependent low temperature kinetics for CN + CH3CN: competition between chemical reaction and van der Waals complex formation

2016 ◽  
Vol 18 (22) ◽  
pp. 15118-15132 ◽  
Author(s):  
Chantal Sleiman ◽  
Sergio González ◽  
Stephen J. Klippenstein ◽  
Dahbia Talbi ◽  
Gisèle El Dib ◽  
...  
Keyword(s):  
Low Temperature ◽  
Complex Formation ◽  
Gas Phase ◽  
Rate Coefficient ◽  
Flow Reactor ◽  
Slow Flow ◽  
Phase Reaction ◽  
Gas Phase Reaction ◽  
Low Temperature Kinetics ◽  
Temperature And Pressure

The gas phase reaction between the CN radical and acetonitrile CH3CN was investigated experimentally with a CRESU apparatus and a slow flow reactor as well as theoretically to explore the temperature and pressure dependence of its rate coefficient from 354 K down to 23 K.

scholarly journals The low temperature D+ + H2 → HD + H+ reaction rate coefficient: a ring polymer molecular dynamics and quasi-classical trajectory study

2018 ◽  
Vol 20 (41) ◽  
pp. 26752-26763 ◽  
Author(s):  
Somnath Bhowmick ◽  
Duncan Bossion ◽  
Yohann Scribano ◽  
Yury V. Suleimanov
Keyword(s):  
Molecular Dynamics ◽  
Low Temperature ◽  
Low Temperatures ◽  
Reaction Rate ◽  
Rate Coefficient ◽  
Classical Trajectory ◽  
Reaction Rate Coefficient ◽  
Ring Polymer ◽  
Ring Polymer Molecular Dynamics ◽  
Trajectory Methods

The D+ + H2 → HD + H+ reaction rate coefficient has been calculated at low temperatures (20–100 K) by ring polymer molecular dynamics and quasi-classical trajectory methods.

scholarly journals Investigations on the gas-phase photolysis and OH radical kinetics of nitrocatechols: Implications of intramolecular interactions on their atmospheric behavior

2021 ◽  
Author(s):  
Claudiu Roman ◽  
Cecilia Arsene ◽  
Iustinian Gabriel Bejan ◽  
Romeo-Iulian Olariu
Keyword(s):  
Gas Phase ◽  
Rate Coefficient ◽  
Relative Rate ◽  
Structural Features ◽  
Intramolecular Interactions ◽  
Rate Coefficients ◽  
Oh Radicals ◽  
Phase Reaction ◽  
Oh Radical ◽  
Atmospheric Conditions

Abstract. The Environmental Simulation Chamber made of Quartz from the University “Alexandru Ioan Cuza” from Iasi (ESC-Q-UAIC), Romania, was used to investigate for the first time the gas-phase reaction rate coefficients for four nitrocatechols towards OH radicals under simulated atmospheric conditions. Employing relative rate technique at a temperature of 298 ± 2 K and total air pressure of 1 atm, the obtained rate coefficients (in 10−12 cm3×s−1) were as followed: k3NCAT = (3.41 ± 0.37) for 3-nitrocatechol, k4NCAT = (1.27 ± 0.19) for 4-nitrocatechol, k5M3NCAT = (5.55 ± 0.45) for 5-methyl-3-nitrocatechol and k4M5NCAT = (0.92 ± 0.14) for 4-methyl-5-nitrocatechol. For the investigated compounds the photolysis rates in the actinic region, scaled to atmospheric relevant conditions, were evaluated as well. In this case the photolysis rate coefficient values were obtained only for 3-nitrocatechol and 5-methyl-3-nitrocatechol: J3NCAT = (3.06 ± 0.16) × 10−4 s−1 and J5M3NCAT = (2.14 ± 0.18) × 10−4 s−1, respectively. Considering the obtained results our study suggests that photolysis may be the main degradation process for 3-nitrocatechol and 5-methyl-3-nitrocatechol in the atmosphere. Results are discussed in terms of the reactivity of the investigated four nitrocatechols towards OH-radical initiated oxidation and their structural features. The rate coefficient values are also compared with those estimated from the structure-activity relationship for monocyclic aromatic hydrocarbons. Additional comparison with similar compounds is also presented underlining the implications towards possible degradation pathways and atmospheric behavior.

scholarly journals Synthetic Approaches to Complex Organic Molecules in the Cold Interstellar Medium

2022 ◽  
Vol 8 ◽  
Author(s):  
Eric Herbst ◽  
Robin T. Garrod
Keyword(s):  
Low Temperature ◽  
Interstellar Medium ◽  
Gas Phase ◽  
Low Temperatures ◽  
Organic Molecules ◽  
High Energy ◽  
Interstellar Space ◽  
Interstellar Clouds ◽  
Millimeter And Submillimeter Waves ◽  
Complex Organic

The observation and synthesis of organic molecules in interstellar space is one of the most exciting and rapidly growing topics in astrochemistry. Spectroscopic observations especially with millimeter and submillimeter waves have resulted in the detection of more than 250 molecules in the interstellar clouds from which stars and planets are ultimately formed. In this review, we focus on the diverse suggestions made to explain the formation of Complex Organic Molecules (COMs) in the low-temperature interstellar medium. The dominant mechanisms at such low temperatures are still a matter of dispute, with both gas-phase and granular processes, occurring on and in ice mantles, thought to play a role. Granular mechanisms include both diffusive and nondiffusive processes. A granular explanation is strengthened by experiments at 10 K that indicate that the synthesis of large molecules on granular ice mantles under space-like conditions is exceedingly efficient, with and without external radiation. In addition, the bombardment of carbon-containing ice mantles in the laboratory by cosmic rays, which are mainly high-energy protons, can lead to organic species even at low temperatures. For processes on dust grains to be competitive at low temperatures, however, non-thermal desorption mechanisms must be invoked to explain why the organic molecules are detected in the gas phase. Although much remains to be learned, a better understanding of low-temperature organic syntheses in space will add both to our understanding of unusual chemical processes and the role of molecules in stellar evolution.

Gas-phase reactivity of CH3C(O) CH3 with OH radicals at interstellar Temperatures (T = 11.7 – 64.4 K) using the CRESU technique

2019 ◽  
Vol 15 (S350) ◽  
pp. 379-381
Author(s):  
Sergio Blázquez ◽  
Antonio J. Ocaña ◽  
Alberto García ◽  
Bernabé Ballesteros ◽  
André Canosa ◽  
...  
Keyword(s):  
Supersonic Flow ◽  
Low Temperature ◽  
Gas Phase ◽  
Pressure Dependence ◽  
Freezing Point ◽  
Rate Coefficients ◽  
Oh Radicals ◽  
Phase Reaction ◽  
Gas Condensation ◽  
Gas Phase Reaction

AbstractThe rate coefficients, k(T= 11.7 – 64.4 K), for the gas-phase reaction between OH radicals and acetone, CH3C(O) CH3, have been measured using the pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique, the most suitable one to cool down gases below the freezing point without gas condensation. The experimental k(T) was found to increase as temperature was lowered and is several orders of magnitude higher for low temperature than k(300 K). No pressure dependence of k(20 K) and k(64 K) was observed, while k(50 K) at the largest gas density is twice higher than the average values found at lower gas densities. The obtained values of k(11.7 K) and k(21.1 K) were 2.45’10-10 and 1.39’10-10 cm3 molecule-1 s-1, respectively.

scholarly journals Reactive quenching of electronically excited NO<sub>2</sub><sup>∗</sup> and NO<sub>3</sub><sup>∗</sup> by H<sub>2</sub>O as potential sources of atmospheric HO<sub><i>x</i></sub> radicals

2018 ◽  
Vol 18 (19) ◽  
pp. 14005-14015 ◽  
Author(s):  
Terry J. Dillon ◽  
John N. Crowley
Keyword(s):  
Gas Phase ◽  
Laser Excitation ◽  
Pulsed Laser ◽  
Work Rate ◽  
Rate Coefficients ◽  
Phase Reaction ◽  
Gas Phase Reaction ◽  
Upper Limit ◽  
Upper Limits ◽  
Electronically Excited

Abstract. Pulsed laser excitation of NO2 (532–647 nm) or NO3 (623–662 nm) in the presence of H2O was used to initiate the gas-phase reaction NO2∗+H2O → products (Reaction R5) and NO3∗+H2O → products (Reaction R12). No evidence for OH production in Reactions (R5) or (R12) was observed and upper limits for OH production of k5b/k5<1×10-5 and k12b/k12<0.03 were assigned. The upper limit for k5b∕k5 renders this reaction insignificant as a source of OH in the atmosphere and extends the studies (Crowley and Carl, 1997; Carr et al., 2009; Amedro et al., 2011) which demonstrate that the previously reported large OH yield by Li et al. (2008) was erroneous. The upper limit obtained for k12b∕k12 indicates that non-reactive energy transfer is the dominant mechanism for Reaction (R12), though generation of small but significant amounts of atmospheric HOx and HONO cannot be ruled out. In the course of this work, rate coefficients for overall removal of NO3∗ by N2 (Reaction R10) and by H2O (Reaction R12) were determined: k10=(2.1±0.1)×10-11 cm3 molecule−1 s−1 and k12=(1.6±0.3)×10-10 cm3 molecule−1 s−1. Our value of k12 is more than a factor of 4 smaller than the single previously reported value.

scholarly journals On apparent barrier-free reactions

2020 ◽  
Vol 3 (1) ◽  
pp. 3-8
Author(s):  
Maikel Ballester
Keyword(s):  
Low Temperature ◽  
Chemical Reactions ◽  
Rate Constant ◽  
Kinetic Models ◽  
Rate Coefficient ◽  
Minimum Energy ◽  
Temperature Limit ◽  
Rate Coefficients ◽  
Minimum Energy Path ◽  
Full Dimension

Rate coefficients of bi-molecular chemical reactions are fundamental for kinetic models. The rate coefficient dependence on temperature is commonly extracted from the analyses of the reaction minimum energy path. However, a full dimension study of the same reaction may suggest a different asymptotic low-temperature limit in the rate constant than the obtained from the energetic profile.

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